Subsea Compression System and Method

ABSTRACT

A subsea hydrocarbon flow compression system ( 100 ) for receiving a hydrocarbon stream from at least one upstream flowline ( 102, 104 ) and supplying the hydrocarbon stream to at least one downstream flowline ( 106, 108 ) at an increased pressure, wherein the compression system comprises first and second compressor trains ( 110   a,    110   b ), wherein each compressor train comprises an inlet port ( 112   a,    112   b ) which is connectable to the at least one upstream flowline ( 102, 104 ); an outlet port ( 114   a,    114   b ) which is connectable to the at least one downstream flowline ( 106, 108 ); a conditioning unit ( 116   a,    116   b ) which is connected to the inlet port via a first flowline ( 118   a,    118   b ); and a first flow path for the hydrocarbon fluid comprising a compressor ( 120   a,    120   b ), which compressor is connected to the conditioning unit via a second flowline ( 122   a,    122   b ) and to the outlet port via a third flowline ( 124   a,    124   b ), wherein a controllable first valve ( 126 ) is arranged in the third flowline of the first compressor train for controlling hydrocarbon flow from the compressor to the outlet port of the first compressor train. A controllable second valve ( 128 ) is arranged in the second flowline of the second compressor train for controlling hydrocarbon flow from the conditioning unit to the compressor of the second compressor train. The system further comprises a first cross-over flowline ( 130 ) interconnecting the third flowline of the first compressor train upstream of the first valve and the second flowline of the second compressor train downstream of the second valve, wherein a controllable first cross-over valve ( 132 ) is arranged in the first cross-over flowline for controlling hydrocarbon flow through the first cross-over flowline.

FIELD OF THE INVENTION

The present invention relates to a subsea hydrocarbon compressionsystem.

In particular, the present invention relates to a subsea hydrocarbonflow compression system for receiving a hydrocarbon stream from at leastone upstream flowline and supplying the hydrocarbon stream to at leastone downstream flowline at an increased pressure, wherein thecompression system comprises first and second compressor trains, whereineach compressor train comprises:

-   -   an inlet port which is connectable to the at least one upstream        flowline;    -   an outlet port which is connectable to the at least one        downstream flowline;    -   a conditioning unit which is connected to the inlet port via a        first flowline; and    -   a first flow path for the hydrocarbon fluid comprising a        compressor, which compressor is connected to the conditioning        unit via a second flowline and to the outlet port via a third        flowline,

wherein a controllable first valve is arranged in the third flowline ofthe first compressor train for controlling hydrocarbon flow from thecompressor to the outlet port of the first compressor train.

The present invention also relates to a method of bringing a subseahydrocarbon flow compression system from a parallel operating mode to aserial operating mode, which compression system is configured to receivea hydrocarbon stream from at least one upstream flowline and supplyingthe hydrocarbon stream to at least one downstream flowline at anincreased pressure, and which compression system comprises first andsecond compressor trains.

BACKGROUND

In subsea hydrocarbon production and processing systems, there may be aneed to “boost” hydrocarbon well-streams, i.e. to increase pressure ofthe hydrocarbon well-stream, e.g. to compensate for pressure losses inflowlines.

GB 2503927 A shows as a method and apparatus for removing hydrate plugsin a hydrocarbon production station. The method comprises the steps offluidically isolating the production station by means of valves,diverting production flow to a bypass line, and adjusting the pressurein the production station to a level sufficient to melt the hydrateplugs by means of a pump/compressor unit and a cooler unit. Hydrateinhibitor such as methanol may be also bled into the system via a line.

FIG. 1 shows a prior art subsea hydrocarbon flow compression systemhaving two compressor trains 10 a, 10 b which can operate in parallel orin series.

When operating in parallel, a multiphase well-stream enters a fluidconditioning unit 12 a, 12 b of each compressor train 10 a, 10 b throughone or more flowlines 14. In FIG. 1 the conditioning units 12 a, 12 bare liquid/gas separators 12 a, 12 b. In the separator 12 a, 12 b, gasand liquid, e.g. a condensate/water/MEG, is separated, and gas is led toa subsea compressor 16 a, 16 b where it is boosted. The liquid is led toand boosted by a pump 18 a, 18 b. Normally the liquid is commingled withthe gas downstream of the compressor 16 a, 16 b for multiphase transportto a receiving facility (not shown).

The system shown in FIG. 1 is a dry-gas system due to the separators 12a, 12 b being arranged to supply the compressors 16 a, 16 b with a fluidstream in which liquid phase components have been removed.

Each compression train 10 a, 10 b may have coolers, e.g. an inlet cooler20 a, 20 b arranged upstream of the compressor 16 a, 16 b and/or anoutlet cooler (not shown) arranged downstream of the compressor 16 a, 16b. An anti-surge cooling functionality may be implemented in the inletcooler 20 a, 20 b and/or in the outlet cooler, and/or in a separateanti-surge cooler (not shown). If an inlet cooler 20 a, 20 b is used,this cooler is always located upstream of the separator 12 a, 12 b toenhance knock-out of liquid in the separator 12 a, 12 b.

The conventional method of switching from parallel to serial operationis to route the gas out of the compressor 16 a of the first compressortrain 10 a and into the inlet of the second compressor train 10 b, e.g.by closing valves 22 and 24, and opening valve 26. In this manner, allthe equipment in the system will be configured in series. In seriesconfiguration, the multiphase well-stream is separated in the firstcompressor train 10 a and the liquid and gas phases are commingled priorto entering the second compressor train 10 b. In the second compressortrain 10 b, the liquid and gas phases are again separated in separator20 b and again commingled prior to entering downstream flowline(s) 28.

In this system, the repeated separation and mixing of phases is a resultof the arrangement and adds no functional benefits.

While the flow is divided between the separators 12 a, 12 b when thecompressor trains 10 a, 10 b are operating in parallel, the separators12 a, 12 b must be dimensioned for the full well-stream when thecompressor trains 10 a, 10 b are operating in series. Althoughhydrocarbon massflow declines over the years prior to series operation(series operation is usually employed during the later productionstages), actual volume rates can still be very high because of thedeclining pressure. In case of slugs or liquid surges, the separator 12a of the first compressor train 10 a must be able to handle the fullslug/surge in series operation, while the slugs will be shared betweenthe separators 12 a, 12 b in parallel operation.

Normally, pumps have much higher pressure boosting capability thancompressors, and series configuration is not required. Consequently,when the system operates in series the separator 12 b and the pump 18 bof the second compressor train 10 b do not provide functionaladvantages, but instead contribute to critical failure modes for thesystem.

FIG. 2 shows another prior art subsea compression station having twocompressor trains 10 a, 10 b which can be switched from operating inparallel to operating in series.

In this case, the compression station incorporates no separators.Instead, in each compressor train 10 a, 10 b a conditioning unit 12 a,12 b in the form of a flow-conditioning device is arranged upstream ofthe compressor 16 a, 16 b to condition the liquid/gas mixture such thatit complies with the requirements of the compressor 16 a, 16 b.

Consequently, the system shown in FIG. 2 is a wet-gas system in whichthe compressors 16 a, 16 b need to boost liquid phase components as wellas gas phase components.

Similar to the system shown in FIG. 1, switching from parallel to serialoperation mode involves routing the discharge of the first compressortrain 10 a into the inlet of the second compressor train 10 b by closingvalves 22 and 24, and opening valve 26.

In serial operation, the same drawback associated with the separator 12b in FIG. 1 is also present for the flow-conditioning device 12 b inFIG. 2, i.e. that the flow-conditioning device 12 b does not give anyfunctional advantage in serial operation mode, but only constitutespossible points of failure in the system.

Surface compression plants are normally not arranged as separate trainswith dedicated processing equipment incorporated in each train. Atypical surface compression plant consists of an upstream processingsystem providing pre-processed gas to a downstream compression system.Switching from parallel to series is done by re-arranging pipes betweenthe compressors, and the upstream processing system is not affected.

Normally, a surface compression system is made for operating either inparallel or in series, and switching of operating mode from parallel toseries is rare. If, however, switching from parallel to series isrequired, this is done by on-site re-build of the pipe-arrangement ofthe system. For surface compression system, this can be done at anacceptable cost and usually requires limited production down-time. Insubsea applications, however, any on-site re-build of the system will beprohibitively expensive and will also require a non-acceptableproduction down-time. Hence, a subsea compression system must bedesigned for all foreseen operation modes with a minimum need of manualintervention.

SUMMARY OF THE INVENTION

With the abovementioned challenges and known solutions in mind, thepresent invention brings forward a subsea hydrocarbon flow compressionsystem and an associated method which seek to solve or at least reduceat least one of the aforementioned problems or challenges.

According to one aspect, the invention relates to a subsea hydrocarbonflow compression system for receiving a hydrocarbon stream from at leastone upstream flowline and supplying the hydrocarbon stream to at leastone downstream flowline at an increased pressure, wherein thecompression system comprises first and second compressor trains, whereineach compressor train comprises:

-   -   an inlet port which is connectable to the at least one upstream        flowline;    -   an outlet port which is connectable to the at least one        downstream flowline;    -   a conditioning unit which is connected to the inlet port via a        first flowline; and    -   a first flow path for the hydrocarbon fluid comprising a        compressor, which compressor is connected to the conditioning        unit via a second flowline and to the outlet port via a third        flowline,

wherein a controllable first valve is arranged in the third flowline ofthe first compressor train for controlling hydrocarbon flow from thecompressor to the outlet port of the first compressor train, and whereina controllable second valve is arranged in the second flowline of thesecond compressor train for controlling hydrocarbon flow from theconditioning unit to the compressor of the second compressor train, andwherein the system comprises:

-   -   a first cross-over flowline interconnecting the third flowline        of the first compressor train upstream of the first valve and        the second flowline of the second compressor train downstream of        the second valve, wherein a controllable first cross-over valve        is arranged in the first cross-over flowline for controlling        hydrocarbon flow through the first cross-over flowline.

Consequently, each compressor train comprises an inlet port, aconditioning unit arranged downstream of the inlet port, a compressorarranged downstream of the conditioning unit, and an outlet portarranged downstream of the compressor. The flow compression system canbe operated in a parallel operating mode in which the hydrocarbon flow,in each compressor train, is routed from the inlet port to the outletport via the conditioning unit and the compressor in respectivecompressor train. The flow compression system can alternatively beoperated in a series operating mode in which the hydrocarbon flow may berouted from the inlet port of the first compressor train to the outletport of the second compressor train via the conditioning unit and thecompressor of the first compressor train and, by virtue of the firstcross-over flowline, via the compressor of the second compressor train.

The system may comprise a second cross-over flowline interconnecting thesecond flowline of the first compressor train and the second flowline ofthe second compressor train upstream of the second valve, wherein acontrollable second cross-over valve may be arranged in the secondcross-over flowline for controlling hydrocarbon flow through the secondcross-over flowline. This allows the hydrocarbon flow to be routed fromthe conditioning unit of the first compressor train to the compressor ofthe second compressor train, or from the conditioning unit of the secondcompressor train to the compressor of the first compressor train.

The system may comprise a third cross-over flowline interconnecting thethird flowline of the first compressor train downstream of the firstvalve and the third flowline of the second compressor train, wherein acontrollable third cross-over valve may be arranged in the thirdcross-over flowline for controlling hydrocarbon flow through the thirdcross-over flowline.

At least one of said first and second compressor trains may comprise aninlet cooler arranged downstream of the conditioning unit and upstreamof the compressor.

The second compressor train may comprise an inlet cooler arrangeddownstream of the conditioning unit and upstream of the compressor,wherein the inlet cooler of the second compressor train may be arrangeddownstream of said second valve, and wherein said first cross-overflowline may be connected to said second flowline of the secondcompressor train upstream of the inlet cooler.

At least one of said first and second compressor trains may comprise atleast one of:

-   -   an outlet cooler arranged downstream of the compressor; and    -   an anti-surge cooler arranged in an anti-surge feed-back loop of        the compressor.

An anti-surge cooling functionality may be embedded in any one of theinlet cooler and the outlet cooler.

Each compressor train may comprise:

-   -   a second flow path for the hydrocarbon fluid arranged in        parallel to said first flow path, which second flow path may        comprise a pump which is connected to the conditioning unit via        a fourth flowline and to the outlet port via a fifth flowline,

wherein said conditioning unit may comprise a separator for separating amultiphase hydrocarbon stream received by the conditioning unit into afirst sub-stream comprising predominately a gaseous fluid phase and asecond sub-stream comprising predominately a liquid fluid phase,

wherein said first flow path may be configured to receive the firstsub-stream from the conditioning unit, and the second flow path may beconfigured to receive the second sub-stream from the conditioning unit,

wherein a controllable third valve may be arranged in the fifth flowlineof the first compressor train for controlling hydrocarbon flow from thepump to the outlet port of the first compressor train, and

wherein the system may comprise:

-   -   a fourth cross-over flowline interconnecting the fifth flowline        of the first compressor train upstream of the third valve and        the third flowline of the second compressor train, wherein a        controllable fourth cross-over valve may be arranged in the        fourth cross-over flowline for controlling hydrocarbon flow        through the fourth cross-over flowline.

The system may comprise a fifth cross-over flowline interconnecting thefourth flowline of the first compressor train and the fourth flowline ofthe second compressor train, wherein a controllable fifth cross-overvalve may be arranged in the fifth cross-over flowline for controllinghydrocarbon flow through the fifth cross-over flowline. Also, a pumpisolation valve may be arranged immediately upstream of each pump. Thiswill allow the liquid fraction of the fluid emerging from the separatorof the conditioning unit of the first compressor train to be led to pumpof the second compressor train, and the fluid emerging from theseparator of the conditioning unit of the second compressor train to beled to the pump of the first compressor train, thus allowing the pump ofthe first compressor train or the pump of the second compressor train tobe disconnected from the fluid path and thus isolated from fluid flow.

Said inlet port of the first compressor train and said inlet port of thesecond compressor train may form a common inlet port and/or said outletport of the first compressor train and said outlet port of the secondcompressor train may form a common outlet port.

Said conditioning unit of each compressor train may be any one of:

-   -   a separator for separating a multiphase hydrocarbon stream into        a first sub-stream comprising predominately a gaseous fluid        phase and a second sub-stream comprising predominately a liquid        fluid phase; and    -   a flow-conditioning device for conditioning the liquid/gas ratio        of the fluid such that it complies with the requirements of the        compressor.

The compression system may have two, three, four or more compressortrains, of which at least two compressor trains are interconnectedaccording to the invention to allow switching between said paralleloperating mode and said serial operating mode.

According to another aspect, the invention relates a method of bringinga subsea hydrocarbon flow compression system from a parallel operatingmode to a serial operating mode, which compression system is configuredto receive a hydrocarbon stream from at least one upstream flowline andsupply the hydrocarbon stream to at least one downstream flowline at anincreased pressure, and which compression system comprises first andsecond compressor trains, wherein each compressor train comprises:

-   -   a conditioning unit which, in said parallel operating mode,        receives hydrocarbon fluid from an inlet port connected to the        at least one upstream flowline; and    -   a first flow path for the hydrocarbon fluid comprising a        compressor which, in said parallel operating mode, receives        hydrocarbon fluid from the conditioning unit and supplies        hydrocarbon fluid to an outlet port connected to the at least        one downstream flowline;

wherein the method comprises the steps of:

-   -   closing a conduit path for the hydrocarbon fluid from the        compressor to the outlet port of the first compressor train;    -   closing a conduit path for the hydrocarbon fluid from the        conditioning unit to the compressor of the second compressor        train; and    -   opening a conduit path for the hydrocarbon fluid from the        compressor of the first compressor train to the compressor of        the second compressor train.

This will allow hydrocarbon fluid to be supplied to the compressor ofthe first compressor train from the conditioning unit of the firstcompressor train but not from the conditioning unit of the secondcompressor train.

The method may comprise the step of opening a conduit path for thehydrocarbon fluid from the conditioning unit of the second compressortrain to the compressor of the first compressor train, thus allowing, inthe serial operating mode, the conditioning units of the first andsecond compressor trains to supply hydrocarbon fluid to the compressorof the first compressor train in parallel.

The method may comprise the steps of:

-   -   opening a conduit path for the hydrocarbon fluid from the        conditioning unit of the second compressor train to the        compressor of the first compressor train; and    -   closing a conduit path for the hydrocarbon fluid from the        conditioning unit of the first compressor train to the        compressor of the first compressor train,

thus allowing, in the serial operating mode, hydrocarbon fluid to besupplied to the compressor of the first compressor train from theconditioning unit of the second compressor train but not from theconditioning unit of the first compressor train (110 a)

The method may comprise the step of opening a conduit path for thehydrocarbon fluid from the compressor of the second compressor train tothe outlet port of the first compressor train.

The method may comprise the step of, in at least one of said compressortrains, routing the hydrocarbon fluid through an inlet cooler arrangeddownstream of the conditioning unit and upstream of the compressor.

The method may comprise the step of, in the second compressor train,routing the hydrocarbon fluid through an inlet cooler arrangeddownstream of the conditioning unit and upstream of the compressor inthe second compressor train, wherein the inlet cooler of the secondcompressor train may be arranged downstream of said second valve, andwherein said first cross-over flowline may be connected to said secondflowline of the second compressor train upstream of the inlet cooler.

The method may comprise the step of, in at least one of said compressortrains, routing the hydrocarbon fluid through at least one of:

-   -   an outlet cooler arranged downstream of the compressor; and    -   an anti-surge cooler arranged in an anti-surge feed-back loop of        the compressor.

Each compressor train may comprise a second flow path comprising a pumpwhich, in said parallel operating mode, receives hydrocarbon fluid fromthe conditioning unit and supplies hydrocarbon fluid to the outlet port,wherein the conditioning unit separates an incoming multiphasehydrocarbon fluid into a first sub-stream comprising predominately agaseous fluid phase and a second sub-stream comprising predominately aliquid fluid phase, which first sub-stream is routed to the outlet portvia the first flow path, and which second sub-stream is routed to theoutlet port via the second flow path, wherein said method may comprisethe steps of:

-   -   closing a conduit path for the hydrocarbon fluid from the pump        to the outlet port in the first compressor train; and    -   opening a conduit path for the hydrocarbon fluid from the pump        of the first compressor train to the outlet port of the second        compressor train.

The method may comprise the step of:

-   -   closing a conduit path for the hydrocarbon fluid from the        conditioning unit of the first compressor train to the pump of        the first compressor train; and    -   opening a conduit path for the hydrocarbon fluid from the        conditioning unit of the first compressor train to the pump of        the second compressor train.

This will allow the pump of the first compressor train to be isolatedfrom fluid flow.

Alternatively, the method may comprise the step of:

-   -   closing a conduit path for the hydrocarbon fluid from the        conditioning unit of the second compressor train to the pump of        the second compressor train; and    -   opening a conduit path for the hydrocarbon fluid from the        conditioning unit of the second compressor train to the pump of        the first compressor train.

This will allow the pump of the second compressor train to be isolatedfrom fluid flow.

According to one aspect, the invention lies in how the crossover betweenthe compressor trains is implemented when the subsea hydrocarbon flowcompression system is switched from parallel to serial operation mode.

When in parallel operation mode, the subsea hydrocarbon flow compressionsystem according to the invention generally operates in the same way asprior art systems.

However, when in serial operation mode, the implementation of thecross-over will enable the compressors (and their anti-surge loops, ifpresent) to operate in series, while allowing the conditioning units andpumps (if present) to continue to operate in parallel.

Also, in some embodiments of the invention, the implementation of thecross-over will allow superfluous conditioning units and/or pump(s) (ifpresent) to be isolated when the system is in serial operating mode (byisolation valves normally provided for each main component), and toserve as installed spares.

This brings about a number of benefits:

-   1. The size and/or rating of the conditioning units and/or pumps can    be reduced because the flow and liquid slugs or surges can be shared    between multiple conditioning units and/or pumps working in parallel    (although the compressors operate in series). Alternatively, in    cases when a conditioning unit and/or pump is not required in serial    operating mode (e.g. due to reduced late life demand), such a unit    can be isolated to serve as an installed spare.-   2. It allows conditioning units and/or pumps that are not    functionally required during serial operating mode to be isolated    from the serial flow path of the hydrocarbon fluid, thus avoiding    equipment in the flow path which are not functionally required and    which could cause loss of production in case of failure.-   3. In systems that incorporate pumps it is possible to maintain, in    serial operating mode, multiple parallel liquid streams through the    system and enable flexible routing of liquid to the downstream    flowlines, keeping control on flow-assurance issues (e.g. MEG    inhibition of flowlines).

Said compressors may be any type compressor suitable to boost, i.e. tocompress and increase pressure of, a hydrocarbon fluid. Such compressorsare known in the art and the details of which will not be discussed atany depth here.

Said pumps may be any type pump suitable to boost, i.e. increase thepressure of, a hydrocarbon fluid. Such pumps are known in the art andthe details of which will not be discussed at any depth here.

Said conditioning units may be any one of a liquid/gas separator forseparating a gas fraction and a liquid fraction from an incominghydrocarbon fluid, and a flow conditioning device (FCD) configured tocondition an incoming liquid/gas mixture of the hydrocarbon fluid suchthat it complies with the requirements of a downstream compressor. Suchconditioning units are known in the art and the details of which willnot be discussed at any depth here.

Said coolers may be any type suitable to cool a hydrocarbon stream. Suchcooler are known in the art and the details of which will not bediscussed at any depth here.

Above-discussed preferred and/or optional features of each aspect of theinvention may be used, alone or in appropriate combination, in the otheraspects of the invention.

In the following, one or more specific embodiments of the invention willbe described in more detail with reference to the drawings.

DESCRIPTION OF THE DRAWINGS

Following drawings are appended to facilitate the understanding of theinvention:

FIG. 1 shows a prior art dry-gas subsea hydrocarbon flow compressionsystem.

FIG. 2 shows a prior art wet-gas subsea hydrocarbon flow compressionsystem.

FIG. 3 shows an embodiment of a wet-gas subsea hydrocarbon flowcompression system according to the invention.

FIG. 4 shows an embodiment of a dry-gas subsea hydrocarbon flowcompression system according to the invention.

It should be understood, however, that the drawings are not intended tolimit the invention to the subject-matter depicted in the drawings.

In the drawings, like reference numerals have been used to indicatecommon parts, elements or features unless otherwise explicitly stated orimplicitly understood by the context.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3 shows an embodiment of a hydrocarbon flow compression system 100according to the invention. The system 100 receives a hydrocarbon fluidstream from upstream flowlines 102, 104 and supplies the hydrocarbonfluid stream to downstream flowlines 106, 108 at an increased pressure.

The system 100 comprises a first compressor train 110 a and a secondcompressor train 110 b. Each compressor train 110 a, 110 b comprises aninlet port 112 a, 112 b which is connected to the upstream flowlines102, 104 for receiving the hydrocarbon fluid from the same. Eachcompressor train 110 a, 110 b also comprises an outlet port 114 a, 114 bwhich is connected to the downstream flowlines 106, 108 for supplyingthe hydrocarbon fluid to the same. Each compressor train 110 a, 110 bfurther comprises a fluid conditioning unit 116 a, 116 b which isconnected to the inlet port 112 a, 112 b via a first flowline 118 a, 118b, and a compressor 120 a, 120 b which is connected to the fluidconditioning unit 116 a, 116 b via a second flowline 122 a, 122 b and tothe outlet port 114 a, 114 b via a third flowline 124 a, 124 b.

Consequently, the first flow line 118 a, 118 b, the fluid conditioningunit 116 a, 116 b, the second flowline 122 a, 122 b, the compressor 120a, 120 b and the third flowline 124 a, 124 b provide a flow path for thehydrocarbon fluid through each compressor train 110 a, 110 b.

Each fluid conditioning unit 116 a, 116 b comprises a flow conditioningdevice (FCD) which is configured to condition the liquid/gas mixture ofthe hydrocarbon fluid such that it complies with the requirements of thecompressor 116 a, 116 b. FCD:s are known as such and will not bedescribed further here. For the purpose of the invention, any prior artFCD can be used in the system shown in FIG. 3 to condition thehydrocarbon fluid such that it complies with the requirements of thecompressor 116 a, 116 b.

Inlet valves 162, 164, 166 are arranged upstream of the inlet ports 112a, 112 b to control routing of the hydrocarbon fluid from the upstreamflowlines 102, 104 into the different compressor trains 110 a, 110 b.Inlet valve 164 can alternatively be substituted with a small-bore pipefor equalize pressure between the inlet ports 112 a and 112 b. Also,outlet valves (not shown) may be arranged downstream of the outlet ports114 a, 114 b to control routing of the hydrocarbon fluid from the outletports 114 a, 114 b to the downstream flowlines 106, 108.

A controllable first valve 126 is arranged in the third flowline 124 aof the first compressor train 110 a for controlling hydrocarbon fluidflow from the compressor 120 a to the outlet port 114 a. Also, acontrollable second valve 128 is arranged in the second flowline 122 bof the second compressor train 110 b for controlling hydrocarbon flowfrom the conditioning unit 116 b to the compressor 120 b.

Furthermore, a first cross-over flowline 130 interconnecting the thirdflowline 124 a of the first compressor train 110 a upstream of the firstvalve 126 and the second flowline 122 b of the second compressor train110 b downstream of the second valve 128, and a controllable firstcross-over valve 132 is arranged in the first cross-over flowline 130for controlling hydrocarbon fluid flow through the first cross-overflowline 130.

When the system 100 operates in parallel mode, the first cross-overvalve 132 is closed and valves 126 and 128 are open. Consequently, inthis operating mode there are two parallel flow paths for thehydrocarbon fluid, i.e. the flow path created through each compressortrain 110 a, 110 b by the first flow line 118 a, 118 b, the fluidconditioning unit 116 a, 116 b, the second flowline 122 a, 122 b, thecompressor 120 a, 120 b and the third flowline 124 a, 124 b.

When switching from parallel to serial operation mode, the first valve126 is closed, thus closing the conduit path for the hydrocarbon fluidfrom the compressor 120 a to the outlet port 114 a in the firstcompressor train 110 a. Also, the second valve 128 is closed, thusclosing the conduit path for the hydrocarbon fluid from the conditioningunit 116 b to the compressor 120 b in the second compressor train 110 b.Furthermore, the first cross-over valve 132 is opened, thus opening aconduit path for the hydrocarbon fluid from the compressor 120 a of thefirst compressor train 110 a to the compressor 120 b of the secondcompressor train 110 b.

Consequently, when switching from parallel to serial operating mode, aconduit path for the hydrocarbon fluid is created through theconditioning unit 116 a and the compressor 120 a of the first compressortrain 110 a, and through the compressor 120 b of the second compressortrain 110 b without routing the hydrocarbon fluid through theconditioning unit 116 b of the second compressor train 110 b.

The system 100 may comprise a second cross-over flowline 134interconnecting the second flowline 122 a of the first compressor train110 a and the second flowline 122 b of the second compressor train 110 bupstream of the second valve 128, and a controllable second cross-overvalve 136 may be arranged in the second cross-over flowline 134 forcontrolling hydrocarbon flow through the second cross-over flowline 134.This will allow the conditioning units 116 a, 116 b of the first andsecond compressor trains 110 a, 110 b to be operated in parallel alsowhen the system 100 is otherwise operating in serial operating mode,i.e. with the compressors 120 a and 120 b operating in series (due tovalves 126 and 128 being closed and valve 132 being open). In otherwords, when the second cross-over valve 136 is open, there will be aparallel fluid path for the hydrocarbon fluid through the conditioningunits 116 a and 116 b to the compressor 120 a, and thereafter a serialfluid path through the compressors 120 a, 120 b, thus allowing, in theserial operating mode, the conditioning units 116 a, 116 b of the firstand second compressor trains 110 a, 110 b to supply hydrocarbon fluid tothe compressor 120 a of the first compressor train 110 a in parallel.

Alternatively, the system may comprise valves (not show) that allow theconditioning unit 116 a of the first compressor train 110 a to beisolated from the inlet ports 12 a, 112 b, thus allowing the system, inserial operating mode, to be operated with both conditioning units 116 aand 116 b in parallel or with only one of the conditioning units 116 a,116 b supplying fluid to the compressors 120 a, 120 b. This will allowany one of the conditioning units 116 a, 116 b to be disconnected fromthe fluid path when the system is operated in serial operating mode.

The system 100 may comprise a third cross-over flowline 138interconnecting the third flowline 124 a of the first compressor train110 a downstream of the first valve 126 and the third flowline 124 b ofthe second compressor train 110 b, and a controllable third cross-overvalve 140 may be arranged in the third cross-over flowline 138 forcontrolling hydrocarbon flow through the same. This allows a conduitpath for the hydrocarbon fluid to be opened from the compressor 120 b ofthe second compressor train 110 b to the outlet port 114 a of the firstcompressor train 110 a, thus allowing fluid to be routed to the outletport 114 a of the first compressor train 110 a also when the system 100is operating in the serial operation mode, i.e. when valve 126 isclosed.

At least one of said first and second compressor trains 110 a, 110 b maycomprises an inlet cooler 142 a, 142 b arranged downstream of theconditioning unit 116 a, 116 b and upstream of the compressor 120 a, 120b.

If the second compressor train 110 b comprises an inlet cooler 142 barranged downstream of the conditioning unit 116 b and upstream of thecompressor 120 b, then the inlet cooler 142 b of the second compressortrain 110 b is advantageously arranged downstream of said second valve128, and said first cross-over flowline 130 is advantageously connectedto said second flowline 122 b of the second compressor train 110 bupstream of the inlet cooler 142 b. In this way, the inlet cooler 142 bwill be in the fluid path of the hydrocarbon fluid also when the system100 is operated in the serial operation mode. However, the firstcross-over flowline 130 may alternatively be connected to the secondflowline 122 b downstream of the inlet cooler 142 b, in which case theinlet cooler 142 b will be in the fluid path of the hydrocarbon fluidonly when the system 100 is operated in parallel operation mode.

In addition to arranging inlet coolers 142 a, 142 b in at least one ofsaid first and second compressor trains 110 a, 110 b, or as analternative thereto, at least one of said first and second compressortrains 110 a, 110 b may comprise at least one of an outlet cooler 144 a,144 b arranged downstream of the compressor 120 a, 120 b; and ananti-surge cooler 146 a, 146 b arranged in an anti-surge feed-back loop148 a, 148 b of the respective compressor 120 a, 120 b.

FIG. 4 shows an embodiment of a dry-gas hydrocarbon flow compressionsystem 100′ according to the invention. The system 100′ comprises thesame features as previously discussed with reference to FIG. 3 and likereference numerals have been used to indicate common parts, elements orfeatures unless otherwise stated.

However, the system 100′ of FIG. 4 differs from the system 100 of FIG. 3in that each compressor train 110 a, 110 b comprises a second flow pathfor the hydrocarbon fluid arranged in parallel to said first flow path,which second flow path comprises a pump 150 a, 150 b which is connectedto the conditioning unit 116 a, 116 b via a fourth flowline 152 a, 152 band to the outlet port 114 a, 114 b via a fifth flowline 154 a, 154 b.Also, in this embodiment the conditioning unit 116 a, 116 b in eachcompressor train 110 a, 110 b comprises a separator for separating amultiphase hydrocarbon stream received by the conditioning unit 116 a,116 b into a first sub-stream comprising predominately a gaseous fluidphase and a second sub-stream comprising predominately a liquid fluidphase. The first flow path, i.e. comprising the compressor 120 a, 120 b,is configured to receive the first, gaseous sub-stream from theconditioning unit 116 a, 116 b, and the second flow path, i.e.comprising the pump 150 a, 150 b, is configured to receive the second,liquid sub-stream from the conditioning unit 116 a, 116 b. Consequently,the system 100′ is a dry-gas hydrocarbon flow compression system.

Also, a controllable third valve 156 is arranged in the fifth flowline154 a of the first compressor train 110 a for controlling hydrocarbonflow from the pump 150 a to the outlet port 114 a of the firstcompressor train 110 a. Furthermore, the system 100′ comprises a fourthcross-over flowline 158 interconnecting the fifth flowline 154 a of thefirst compressor train 110 a upstream of the third valve 156 and thethird flowline 124 b of the second compressor train 110 b, wherein acontrollable fourth cross-over valve 160 is arranged in the fourthcross-over flowline 158 for controlling hydrocarbon flow through thefourth cross-over flowline 158.

When operating in parallel mode, cross-over valves 132, 136, 140 and 160are closed and valves 126, 128 and 156 are open, thus allowinghydrocarbon fluid to flow from the upstream flowlines 102, 104 to thedownstream flowlines 106, 108 in parallel in the first and secondcompressor trains 110 a and 110 b, wherein, in each compressor train 110a, 110 b, the gaseous fraction of the hydrocarbon fluid is boosted inthe compressor 120 a, 120 b and the liquid fraction is boosted in thepump 150 a, 150 b.

When switching from parallel to serial operational mode, the first andsecond valves 126, 128 are closed, and the first and second cross-overvalves 132, 136 are opened. This will close the conduit paths for thehydrocarbon fluid from the compressor 120 a to the outlet port 114 a inthe first compressor train 110 a and from the conditioning unit 116 b tothe compressor 120 b in the second compressor train 110 b, and openconduit paths from the compressor 120 a of the first compressor train110 a to the compressor 120 b of the second compressor train 110 b andfrom the conditioning unit 116 b of the second compressor train 110 b tothe compressor 120 a of the first compressor train 110 a.

The positions (open or closed) of the third valve 156 and the fourthcross-over valve 160 will decide which of the downstream flowlines 106,108 will receive the liquid from the pump 150 a in the first compressortrain 110 a, and the position of the third cross-over valve 140 willdecide whether the gas output from the system will be disposed in asingle flowline 114 b or if the gas will be shared between flowlines 114a and 114 b.

The system 100′ may comprise a fifth cross-over flowline 168interconnecting flowlines 152 a and 152 b, wherein a controllable fifthcross-over valve 170 may be arranged in the fifth cross-over flowlinefor controlling hydrocarbon flow through the same. Also, a pumpisolation valve (not shown) may be arranged immediately upstream of eachpump 150 a and 150 b. This will allow the liquid fraction of the fluidemerging from the separator of conditioning unit 116 a to be led to pump150 b, and the fluid emerging from the separator of conditioning unit116 b to be led to pump 150 a, thus allowing anyone of pumps 150 a and150 b to be disconnected from the fluid path.

In the same way as the system 100 discussed with reference to FIG. 3, atleast one of said first and second compressor trains 110 a, 110 b maycomprises an inlet cooler 142 a, 142 b arranged downstream of theconditioning unit 116 a, 116 b and upstream of the compressor 120 a, 120b.

However, if the system 100′ comprises such an inlet cooler 142 a, 142 b,some liquid may condense in the cooler 142 a, 142 b and enter thecompressor 120 a, 120 b. Consequently, for such an arrangement thecompressor 120 a, 120 b should preferably be tolerant of liquid, i.e. beable to handle that at least some liquid is present in the incomingfluid stream.

In addition or as an alternative to said inlet coolers 142 a, 142 b, atleast one of said first and second compressor trains 110 a, 110 b maycomprise at least one of an outlet cooler 144 a, 144 b arrangeddownstream of the compressor 120 a, 120 b; and an anti-surge cooler 146a, 146 b arranged in an anti-surge feed-back loop 148 a, 148 b of therespective compressor 120 a, 120 b.

In serial operation mode, some liquid may condense in the outlet cooler144 a of the first compressor train 110 a (if present) and/or in theinlet cooler 142 b of the second compressor trains 110 b (if present).Consequently, in such an arrangement the compressor 120 b of the secondcompressors train 110 b should preferably be tolerant of liquid, i.e. beable to handle that at least some liquid is present in the incomingfluid stream.

In the preceding description, various aspects of the system and methodaccording to the invention have been described with reference to theillustrative embodiments. For purposes of explanation, specific numbers,systems and configurations were set forth in order to provide a thoroughunderstanding of the system and its workings. However, this descriptionis not intended to be construed in a limiting sense. Variousmodifications and variations of the illustrative embodiment, as well asother embodiments of the system and method, which are apparent to personskilled in the art to which the disclosed subject-matter pertains, aredeemed to lie within the scope of the present invention as defined bythe following claims.

1. A subsea hydrocarbon flow compression system for receiving a hydrocarbon stream from at least one upstream flowline and supplying the hydrocarbon stream to at least one downstream flowline at an increased pressure, wherein the compression system comprises: first and second compressor trains, each of which comprises: an inlet port which is connectable to the at least one upstream flowline; an outlet port which is connectable to the at least one downstream flowline; a conditioning unit which is connected to the inlet port via a first flowline; and a first flow path for the hydrocarbon fluid, the first flow path comprising a compressor, which is connected to the conditioning unit via a second flowline and to the outlet port via a third flowline; a controllable first valve which is arranged in the third flowline of the first compressor train for controlling hydrocarbon flow from the compressor of the first compressor train to the outlet port of the first compressor train; a controllable second valve which is arranged in the second flowline of the second compressor train for controlling hydrocarbon flow from the conditioning unit of the second compressor train to the compressor of the second compressor train; a first cross-over flowline interconnecting the third flowline of the first compressor train upstream of the controllable first valve and the second flowline of the second compressor train downstream of the controllable second valve; and a controllable first cross-over valve which is arranged in the first cross-over flowline for controlling hydrocarbon flow through the first cross-over flowline.
 2. The system according to claim 1, further comprising: a second cross-over flowline interconnecting the second flowline of the first compressor train and the second flowline of the second compressor train upstream of the second valve; and a controllable second cross-over valve which is arranged in the second cross-over flowline for controlling hydrocarbon flow through the second cross-over flowline.
 3. The system according to claim 1, wherein at least one of said first and second compressor trains comprises an inlet cooler arranged downstream of the conditioning unit and upstream of the compressor.
 4. The system according to claim 3, wherein the second compressor train comprises an inlet cooler arranged downstream of the conditioning unit and upstream of the compressor, wherein the inlet cooler of the second compressor train is arranged downstream of said second valve, and wherein said first cross-over flowline is connected to said second flowline of the second compressor train upstream of the inlet cooler.
 5. The system according to claim 1, wherein each compressor train further comprises: a second flow path for the hydrocarbon fluid arranged in parallel to said first flow path, the second flow path comprising a pump which is connected to the conditioning unit via a fourth flowline and to the outlet port via a fifth flowline; wherein each conditioning unit comprises a multiphase separator for separating a multiphase hydrocarbon stream received by the conditioning unit into a first sub-stream comprising predominately a gaseous fluid phase and a second sub-stream comprising predominately a liquid fluid phase; wherein said first flow path is configured to receive the first sub-stream from the conditioning unit, and the second flow path is configured to receive the second sub-stream from the conditioning unit; and wherein the system further comprises: a controllable third valve which is arranged in the fifth flowline of the first compressor train for controlling hydrocarbon flow from the pump of the first compressor train to the outlet port of the first compressor train; a fourth cross-over flowline interconnecting the fifth flowline of the first compressor train upstream of the controllable third valve and the third flowline of the second compressor train, and a controllable fourth cross-over valve which is arranged in the fourth cross-over flowline for controlling hydrocarbon flow through the fourth cross-over flowline.
 6. The system according to claim 5, further comprising: a fifth cross-over flowline interconnecting the fourth flowline of the first compressor train and the fourth flowline of the second compressor train, and a controllable fifth cross-over valve which is arranged in the fifth cross-over flowline for controlling hydrocarbon flow through the fifth cross-over flowline.
 7. A method of bringing a subsea hydrocarbon flow compression system from a parallel operating mode to a serial operating mode, the compression system is being configured to receive a hydrocarbon stream from at least one upstream flowline and supply the hydrocarbon stream to at least one downstream flowline at an increased pressure, and the compression system comprising: first and second compressor trains each of which comprises: a conditioning unit which, in said parallel operating mode, receives hydrocarbon fluid from an inlet port connected to the at least one upstream flowline; and a first flow path for the hydrocarbon fluid, the first flow path comprising a compressor which, in said parallel operating mode, receives hydrocarbon fluid from the conditioning unit and supplies hydrocarbon fluid to an outlet port connected to the at least one downstream flowline; wherein the method comprises the steps of: closing a conduit path for the hydrocarbon fluid from the compressor of the first compressor train to the outlet port of the first compressor train; closing a conduit path for the hydrocarbon fluid from the conditioning unit of the second compressor train to the compressor of the second compressor train; and opening a conduit path for the hydrocarbon fluid from the compressor of the first compressor train to the compressor of the second compressor train.
 8. The method according to claim 7, wherein hydrocarbon fluid is supplied to the compressor of the first compressor train from the conditioning unit of the first compressor train but not from the conditioning unit of the second compressor train.
 9. The method according to claim 7, further comprising the step of opening a conduit path for the hydrocarbon fluid from the conditioning unit of the second compressor train to the compressor of the first compressor train, thus allowing, in the serial operating mode, the conditioning units of the first and second compressor trains to supply hydrocarbon fluid to the compressor of the first compressor train in parallel.
 10. The method according to claim 7, further comprising the steps of: opening a conduit path for the hydrocarbon fluid from the conditioning unit of the second compressor train to the compressor of the first compressor train; and closing a conduit path for the hydrocarbon fluid from the conditioning unit of the first compressor train to the compressor of the first compressor train; thus allowing, in the serial operating mode, hydrocarbon fluid to be supplied to the compressor of the first compressor train from the conditioning unit of the second compressor train but not from the conditioning unit of the first compressor train.
 11. The method according to claim 7, further comprising the step of, in at least one of said compressor trains, routing the hydrocarbon fluid through an inlet cooler arranged downstream of the conditioning unit and upstream of the compressor.
 12. The method according to claim 11, further comprising the step of, in the second compressor train, routing the hydrocarbon fluid through an inlet cooler arranged downstream of the conditioning unit and upstream of the compressor in the second compressor train, wherein the inlet cooler of the second compressor train is arranged downstream of said second valve, and wherein said first cross-over flowline is connected to said second flowline of the second compressor train upstream of the inlet cooler.
 13. The method according to claim 7, wherein each compressor train comprises a second flow path comprising a pump which, in said parallel operating mode, receives hydrocarbon fluid from the conditioning unit and supplies hydrocarbon fluid to the outlet port, wherein the conditioning unit separates an incoming multiphase hydrocarbon fluid into a first sub-stream comprising predominately a gaseous fluid phase and a second sub-stream comprising predominately a liquid fluid phase, wherein the first sub-stream is routed to the outlet port via the first flow path and the second sub-stream is routed to the outlet port via the second flow path, and wherein the method further comprises the steps of: closing a conduit path for the hydrocarbon fluid from the pump of the first compressor train to the outlet port of the first compressor train; and opening a conduit path for the hydrocarbon fluid from the pump of the first compressor train to the outlet port of the second compressor train.
 14. The method according to claim 13, further comprising the steps of: closing a conduit path for the hydrocarbon fluid from the conditioning unit of the first compressor train to the pump of the first compressor train; and opening a conduit path for the hydrocarbon fluid from the conditioning unit of the first compressor train to the pump of the second compressor train.
 15. The method according to claim 13, further comprising the steps of: closing a conduit path for the hydrocarbon fluid from the conditioning unit of the second compressor train to the pump of the second compressor train; and opening a conduit path for the hydrocarbon fluid from the conditioning unit of the second compressor train to the pump of the first compressor train. 